Preparation of powder metallurgy compositions containing dispersed refractory oxides and precipitation hardening elements

ABSTRACT

NICKEL-CHROMIUM ALLOY COMPOSITIONS CONTAINING DISPERSED REFRACTORY OXIDE PARTICLES AND PRECIPITATION HARDENING ELEMENTS SUCH AS ALUMINUM ARE PREPARED BY BLENDING NICKEL-CHROMIUM ALLOY-REFRACTORY OXIDE POWDER WITH COMPOSITE NICKEL COATED ALUMINUM POWEDER OR NICKELALUMINUM ALLOY POWDER AND, OPTIONALLY, WITH TITANIUM HYDRIDE POWDER. THIS POWDER BLEND IS COMPACTED AND VACUUM SINTERED TO REDUCE THE OXYGEN CONTENT TO LESS THAN 1000 PARTS PER MILLION AND PLACE THE ALUMINUM (AND TITANIUM WHEN PRESENT) IN SOLUTION WITH THE NICKEL-CHROMIUM ALLOY. THE PRODUCT IS SUITABLE FOR PROCESSING IN ACCORDANCE WITH PRIOR ART FABRICATION METHODS TO PRODUCE WROUGHT DISPERSION AND PRECIPITATION STRENGTHENED NICKEL-CHROMIUM ALLOY PRODUCTS.

3,695,868 Patented Oct. 3, 1972 PREPARATION OF POWDER METALLURGY COM- POSITIONS CONTAINING DISPERSED REFRAC- TORY OXIDES AND PRECIPITATION HARDEN- ING ELEMENTS Leon F. Norris, Fort Saskatchewan, Alberta, Canada, assignor to Sherritt Gordon Mines Limited, Toronto, Province of Ontario, Canada No Drawing. Filed June 22, 1970, Ser. No. 48,528

Int. 61. B221? 1/00 U.S. Cl. 75-206 5 Claims ABSTRACT OF THE DISCLOSURE Nickel-chromium alloy compositions containing dispersed refractory oxide particles and precipitation hardening elements such as aluminum are prepared by blending nickel-chromium alloy-refractory oxide powder with composite nickel coated aluminum powder or nickelaluminum alloy powder and, optionally, with titanium hydride powder. This powder blend is compacted and vacuum sintered to reduce the oxygen content to less than 1000 parts per million and place the aluminum (and titanium when present) in solution with the nickel-chromium alloy.

The product is suitable for processing in accordance with prior art fabrication methods to produce wrought dispersion and precipitation strengthened nickel-chromium alloy products.

This invention relates to a process for preparing nickelchromium alloy compositions which contain a dispersion of refractory metal oxide particles and which also contain aluminum and, optionally, titanium in solid solution. The compositions are particularly suited for heat-treating and aging to precipitation-strength them thereby providing the benefits of both the dispersion strengthening and precipitation strengthening mechanisms.

Dispersion strengthening with ultra fine refractory metal oxide particles is a known expedient for improving the strength characteristics of nickel and nickel-chromium alloys at elevated temperatures such as 1600-2200 F. (See for example U.S. Pat. Nos. 3,366,515 and 3,454,431.)

It is desirable to improve the strength of dispersion strengthened nickel-chromium alloys in the intermediate temperature range of 1000-1600 F. and precipitation strengthening provides one possible means of achieving this object. Precipitation-strengthening with gamma prime is a known expedient for improving the strength characteristics of metals at intermediate temperatures such as 1000-1600 F. This mechanism is employed to strengthen certain members of the family of compositions known as super-alloys. In general terms, these compositions comprise a nickel-chromium matrix having Ni (Al, Ti) intermetallic particles distributed therethrough. [In most superalloys, other strengthening mechanisms, such as the incorporation of carbide particles in the matrix are also used.

However, problems arise in the manufacture of precipitation and dipsersion strengthened nickel-chromium alloys by the conventional powder metallurgy methods which involve blending, compacting and sintering of the various alloy constituents in finely divided powder form. When these techniques are used, serious oxygen contamination of the metal constituents occurs particularly when active oxygen scavengers, such as chromium and titanum, are involved.

One result of such oxygen contamination is alteration of the chemical composition of the alloy. Usually, the composition has been carefully designed to give optimum results. If the composition is altered by internal oxidation of one or more of the alloy constituents, the desired precipitation strengthening characteristics are not achieved in the final wrought products regardless of the fabrication and heat treatment procedures employed.

It is clear that the formation of oxides must be minimized if a precipitation and dispersion strengthened nickel-chromium alloy compositions of good quality are to be produced.

It is therefore the primary object of this invention to provide a powder metallurgical process for making nickelchromium alloy compositions containing a uniform dispersion of refractory metal oxide particles as well as aluminum and, optionally, titanium in solid solution and whereby oxygen contamination is minimized.

The first requirement of the invention is the provision of a nickel-chromium alloy powder containing uniformly dispersed, sub-micron refractory oxide particles and having a very low excess oxygen content. Various methods are known for producing such nickel-chromium alloy refractory oxide powders. The method described in copending United States application Ser. No. 813,214 is particularly suitable. The powders produced by this method have a homogeneous nickel-chromium alloy matrix, are oxidation resistant and, therefore, can be handled without taking elaborate precautions to avoid contact with oxygen. The powder contains only a small amount of excess ox'y'gen-about 0.6 by weight or less. (By excess oxygen is meant any oxygen associated with the powder other than that combined in the refractory oxide.)

The nickel-chromium alloy-refractory oxide powder is blended with aluminum and if desired titanium-containing powder, particles which are in substantially nonoxidized form. The aluminum is provided as composite nickel-coated aluminum powder or nickel-aluminum alloy powder and the titanium as titanium hydride.

The powder blend, preferably in the form of a compacted billet, is heated under vacuum to reduce its excess oxygen content to less than 1000 preferably less than 500 parts per million. At the same time, the aluminum (and titanium when present) goes into solid solution with the nickel-chromium alloy. The resulting composition is suitable for fabrication of wrought dispersion strengthened nickel-chromium alloy forms which may be heat treated and aged to cause precipitation strengthening.

There are three critical aspects in the procedure just outlined. Firstly, the chromium content of the composition must be in substantially non-oxidizable nickel-chromium alloy form before it is heated in combination with aluminum and titanium so that there is little oxygen available for these latter metals to getter during the heat treating operation. Aluminum and titanium readily form stable oxides which are very diflicult and expensive to reduce but, according to the method of the invention, large scale formation of these oxides and the need for their subsequent reduction are avoided. Secondly, the aluminum and titanium constituents themselves are provided in substantially non-oxidized form; therefore, the extent of oxide reduction required in the final step is further diminished. In these circumstances, expensive oxide removal techniques, such as vacuum sintering become feasible for removing the residual excess oxygen. Thirdly, the excess oxygen content of the vacuum treated end product must be less than 1000- parts per million.

The invention can be utilized to prepare a wide variety of super-alloy compositions containing nickel, chromium, aluminum and titanium. Thoria is generally preferred as the dispersion strengthening or dispersoid phase because of its high temperature stability and ready availability. However, other refractory oxides which are known to be useful as dispersion strengthening agents, such as yttria, zirconia, alumina, magnesia, etc., can be substituted for the thoria. Additionally, if desired, the titanium can be omitted altogether.

A preferred starting material is nickel-chromium alloy powder prepared by the process described in co-pending application Serial No. 813,214. Briefly, according to this process, nickel refractory oxide powder, preferably as produced in accordance with the teaching of US. Pat. No. 3,469,967, is blended with commercially available chromium powder. Preferably, the chromium powder is fine, in the order of 1-10 microns or less particle size, so as to promote rapid homogenziation of the nickel and chromium. If the chromium particles are smaller than about 1 micron, relatively large amounts of oxide will be associated with them and carried into the alloy. If they are larger than about 10 microns, homogenization of the nickel-chromium alloy is rendered difiicult. The nickelrefractory oxide and chromium powders preferably are blended by slurrying in water and grinding/mixing in an attrition mill or ball mill. The attritor technique is preferred because of the efiiciency of the milling action in promoting a uniform distribution of both the refractory oxide and the chromium in the final powder mixture. The attritor ground mixture of nickel-refractory oxide and chromium powders is next heat treated in pure flowing hydrogen to deoxidize it and simultaneously alloy the nickel and chromium. This is done by heating the mixture to 1800 to 2200 F., preferably to about 2050 F. at a rate of about 200 F. per hour, and then soaking it at such elevated temperature for a prolonged period, such as 20 to 60 hours. The product powder consists of nickelchromium alloy particles with mechanically inseparable refractory oxide particles such as thoria distributed therein. The powder has a very low excess oxygen content, usually less than 1% by weight. Additionally, it is in a highly oxidation resistant form which permits it to be handled in the subsequent processing steps of the invention without risk of oxidation.

According to the invention, the nickel-chromium alloyrefractory oxide powder is blended with particles of aluminum or aluminum and titanium. These precipitation strengthening elements must be provided in a substantially oxide-free, oxidation resistant form. Preferably, the aluminum is provided as nickel-aluminum composite powder produced by the hydrometallurgical technique described in US. Pat. No. 2,853,403. These powders, which consist of an aluminum core particle uniformly coated with a thin layer of nickel, have an inherently low oxygen content and can be handled in the subsequent steps of the process without risk of the aluminum forming stable oxides. The powders are commercially available in various sizes and with various nickel-aluminum ratios. A preferred powder contains 60-70 wt. percent nickel and 30-40 wt. percent aluminum and has an average particle size below about 100 microns. The aluminum may also be provided in the form of a nickel-aluminum alloy powder which is also commercially available. When titanium is added along with the aluminum, it may also be in the form of a composite nickel coated powder prepared in accordance with US. Pat. No. 2,853,403 or in the form of a nickel-titanium alloy powder. However, as a practical economic matter, it is preferred to use titanium hydride as the titanium source material.

The powder constituents preferably are blended and then compacted to form a 60-70% dense billet. The billet is vacuum sintered to lower its excess oxygen content to less than 1000 parts per million and to produce a product which is strong enough to be handled in subsequent fabrication and/or heat-treating procedures. In a preferred procedure, the billet is vacuum sintered by raising its temperature to 2000-2200 F. preferably 2050 F. while maintaining a pressure of 10- torr in the sintering furnace.

The end product from the vacuum sintering can be fabricated to a fully dense wrought product by known methods such as that described in US. Pat. No. 3,366,-

4 515, for example, and then solution treated and aged in accordance with procedures which are well understood in the precipitation-strengthening art.

The invention is more clearly understood with reference to the following examples.

In the examples, the following powder constituents were used:

(1) Nickel-chromium alloy-thoria powder prepared in accordance with procedures described in co-pending application Serial No. 813,214. This powder contained (wt. percent Ni78.1, Cr-l9.7 and ThO -2.2. The apparent density (A.D.) was 1.9 g./cc., Fisher No. 8.2 and excess oxygen content less than 100 ppm.

(2) Nickel-aluminum composite powder prepared in accordance with the method of US. Pat. No. 2,853,403. The powder contained 66 wt. percent nickel, 34 wt. percent aluminum. The AD. was 1.9 g./cc. average particle size 60 microns and oxygen content 1.3%.

(3) Titanium hydridecommercial grade, A. D. 5.1 g./cc., Fisher No. 1.1, oxygen content 0.6%.

(4) Nickel aluminum alloy powder commercial grade, A. D. 1.5 g./cc., Fisher No. 6.2, oxygen content 0.8%.

EXAMPLE I 481 grams of the nickel-chromium alloy-thoria powder, 13 grams of the TiH powder and 18.5 grams of the composite nickel-aluminum powder were dry blended in a high speed blender for 4 minutes. The powder blend was compacted into 30 gram billets in a double-acting die using a pressing pressure of 33 tons/ sq. in. The compacts measured 2.4 in. x 1.25 in. x 0.10 in. The compacted billets were sintered by heating to 2055 F. in 12 hours while maintaining a vacuum of 50 torr during heat-up. The billets were maintained at 2055 F. for 15 hours during which time the vacuum in the system dropped to less than 10- torr. After sintering, the billets had 400 p.p.m. excess oxygen. The billets were fabricated to 0.020 in. strip by the following procedure: A 35% reduction in thickness by cold rolling the sintered billet; heating to 2250 F. in pure H taking a 70% reduction in thickness by hot rolling, and annealing at 2300 F. for 1 hour in pure hydrogen. The strength properties of the annealed strip at 1200 F. were: Y.S. 78,500 p.s.i., UTS-78,500 p.s.i. and elongation (percent in 2 in.) -1.. The annealed strip was heat treated to effect precipitation of the intermetallic Ni (Al, Ti) by the following procedure: Solution treat for 70 hours at 2200 F. in pure hydrogen (time was the minimum required to lower the hardness of the asquenched strip to its minimum value of R45T 60 from an as-fabricated hardness of R45T 73), quench in agitated water, age for 48 hours at 1300 F. in pure hydrogen to effect precipitation (time was the minimum to raise the hardness to its maximum value of R45T 82).

The product had the following characteristics: A dispersion of fine thoria particles (0.02-01 microns), a dispersion of particles of the intermetallic Ni (Al, Ti) as measured from surface replica in electron microscope of 0.01-0.2 microns (the presence of the intermetallic was inferred from electron diffraction analysis of Ni Al), a microstructure containing roughly equi-axed fine grains, grain size was approximately 5-10 microns, and the following short-time mechanical properties:

1 Not determined By way of comparison, the strength properties of conventional Ni/Cr/1.3Al/2.5 Ti alloy, (Nimonic A) according to data from ASM Metals Handbook, vol. I, 8th edition are as follows:

EXAMPLE II In this example, the procedure was the same as that of Example 1 except that 11.2 grams of the nickel-aluminum alloy powder was used in the powder blend in place of the composite nickel-aluminum powder and the sintered billets had 90 p.p.m. excess oxygen. The product had the following characteristics: Microstructural feature similar to those in Example 1 and short-time tensile properties:

El Y.S U.T.S. (percent (p.s.i.) (p.s.i.) in 2 in.)

Test temperature, F.

RT 142, 000 4 1, 200 76, 000 88, 000 2 I, 600 18, 300 19, 400 8 2, 000 6, 400 7, 600 9 1 N or. determined.

In other tests in which essentially the same procedures as those used in Examples I and II were followed but in which the vacuum sintering step was terminated before excess oxygen in the sintered billets was lowered to 1000 p.p.m. or less, there was no effective precipitation of Ni;; (Al, Ti) during the heat treatment steps. In still other tests in which aluminum powder was used in place of composite Ni-Al powder or Ni-Al allo powder, excess could not be reduced to less than 1000 p.p.m. even with prolonged vacuum sintering and, again, there was no effective precipitation of M (Al, Ti) during the heat treatment of such material.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. A method for producing nickel-chromium alloy compositions containing (1) a dispersion of refractory oxide particles and (2) a metallic component capable of forming gamma prime intermetallic compound with nickel upon heat treatment and aging which comprises: providing nickel-chromium alloy powder having uniformly dispersed submicron refractory oxide particles associated therewith and containing less than 1% by weight of oxygen in excess of the oxygen contained in said refractory oxide particles, mixing said nickel-chromium alloy powder with a metallic component selected from the group consisting of composite nickel-coated aluminum powder and nickel-aluminum alloy powder, and vacuum sintering the resulting mixture at an elevated temperature within the range of 2000 F. to 2200 F. for a sufficient period of time to lower the excess oxygen content thereof to less than 1000 parts per million.

2. The method defined in claim 1 wherein the nickelchromium alloy powder is also mixed with finely divided titanium hydride.

3. The method defined in claim 2 wherein the vacuum sintering operation is continued for sulficient time to reduce the excess oxygen content to less than 500 parts per million.

4. The method defined in claim 2 wherein the product from the vacuum sintering step is consolidated to a fully dense wrought form by mechanical working and then heat treated and aged to precipitate a gamma prime phase.

5. The method defined in claim 2 wherein the nickelaluminum powder contains -70 weight percent nickel and 3-40 weight percent aluminum.

References Cited UNITED STATES PATENTS 3,180,727 4/ 1965 Alexander et al. 206

CARL D. QUARFORTH, Primary Examiner B. HUNT, Assistant Examiner US. Cl. X.R. 75225; 148126 

